@khean I've not found out how to do it yet, so I'm not sure it's a thing.
However, using the advanced view on the desktop site, you can search for a hashtag, and then pin the column/window of search results, so you always have it visible.
Click the settings button in the top right of that column and click "Pin". You can pin and unpin different stuff, and even reposition some with arrow buttons to really customise your view.
But yeah, that's not exactly following a hashtag